Synthesis of Oligosaccharide Components of the Outer Core Domain of P. aeruginosa Lipopolysaccharide Using a Multifunctional Hydroquinone-Derived Reducing-End Capping Group

Abstract

The synthesis of a trisaccharide (common to glycoform I and II) and a tetrasaccharide (common to glycoform I) from the outer core domain of Pseudomonas aeruginosa lipopolysaccharide (LPS) using a novel hydroquinone-based reducingend capping group is reported. This multifunctional capping group was utilized as purification handle and was stable toward many common transformations in oligosaccharide synthesis. The access to outer-core LPS antigens with a TBDPS-protected hydroquinone (TPH) at the reducing end will be useful for glycan array and therapeutic glycoconjugate synthesis

Pseudomonas aeruginosa is a Gram-negative bacterium with intrinsic antibiotic resistance. This opportunistic human pathogen is a leading cause of death among cystic fibrosis (CF) patients. P. aeruginosa is also associated with hospital-acquired pneumonia and blood infections. Synthetic outer core domains from the P. aeruginosa lipopolysaccharide (LPS) will be valuable components of protective vaccines since the full isolated LPS is toxic.¹

According to the International Antigenic Typing System (IATS), the strains of P. aeruginosa are classified into 20 different serotypes on the basis of the serological reactivity of the O-polysaccharides.^2,3These O-polysaccharides are attached to Lipid A via an intervening core oligosaccharide. Depending on the presence or absence of O-polysaccharide, these lipopolysaccharides are termed as S (smooth)-type lipopolysaccharides or R (rough)-type lipopolysaccharides, respectively.^4,5The unique feature of all lipopolysaccharides is the presence of structurally similar outer-core glycoform I (Figure 1) and closely related glycoform II. The two glycoforms share monosaccharides, consisting of one d-galactosamine residue, three d-glucose residues, and one l-rhamnose residue.^6–8Antibodies against these oligosaccharides have been shown to be protective against some serotypes of P. aeruginosa.^9,10

Figure 1.

Outer core domain of P. aeruginosa R-type lipopolysaccharide (glycoform I).

The synthesis of oligosaccharides with a protecting group at the reducing end is useful for the purification of single anomers. The reducing-end group is typically removed after synthesis for conjugation with other materials to form arrays or vaccine conjugates. While groups like p-methoxyphenyl can serve this purpose, they are often not amenable to postglycosylation functionalization. Groups like p-nitrophenyl (PNP) can be reduced to p-aminophenyl and coupled with linkers that enable array formatting¹¹. Nonetheless, presence of a PNP group at the reducing end limits the scope of the oligosaccharide chemistry that can be carried out and is generally more compatible with chemoenzymatic methodologies.¹² We envisioned that a latent hydroquinone would be valuable for late-stage conjugation chemistry and would confer better stability during many chemical transformations. Examples of conjugation with hydroquinones include Mitsunobu, α-haloketo, and Mannich-type chemistry.^13,14 In addition, a hydroquinone was expected to be compatible with common oligosaccharide deprotection chemistries, including catalytic hydrogenation and Zn/AcOH reductions, under which PNP is reduced. This innovation provides additional flexibility to leave the reducing-end protecting group in place. We report herein the use of tert- butyldiphenylsilyl (TBDPS)-protected hydroquinone (TPH) 2 as a multifunctional reducing-end capping group and show how it can be used to assemble the outer core domain of P. aeruginosa LPS. Compound 2 was prepared as reported elsewhere (Figure 2).¹⁵

Figure 2.

TBDPS-protected hydroquinone (TPH) 2 as a reducing- end capping group.

Syntheses of some fragments of outer core domains of P. aeruginosa bearing a methoxy group at the reducing end have been reported.^16,17In order to access conjugable outer core domains of P. aeruginosa LPS, we synthesized thioglycoside donor building blocks A–D (Figure 3). We employed fluorenylmethyloxycarbonyl (Fmoc) and levulinate (Lev) protecting groups in the donor building blocks in order to achieve selective deprotection during tri- and tetrasaccharide synthesis. The groups are compatible with emerging automated oligosaccharide synthesis technologies.

Figure 3.

Donor building blocks A–D.

Thioglycoside intermediates 3 and 4 were obtained from d-glucose over six steps according to reported procedures.^18–21 S-4-Methylphenyl-2,3,4-tri-O-benzoyl 1-thio-β-d-glucopyranoside (3) was treated with Fmoc chloride in the presence of pyridine at room temperature to afford building block A in 91% yield (Scheme 1). Similarly, Fmoc protection was achieved with 4-methylphenyl 2,3,4-tri-O-benzyl-1-thio-β-d-glucopyranoside (4) to obtain B in 89% yield. The coupling reaction of 4 with levulinic acid using dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) afforded thioglycoside donor C in 85% yield. The l-rhamnose-based glycosyl donor D was synthesized over eight steps starting from L-rhamnose according to reported literature.^22–24

Scheme 1.

Synthesis of Donor Building Blocks

With the donor building blocks in hand, our synthesis of the common trisaccharide of the two glycoforms was started from 2-azido-2-deoxy-3,4,6-tri-O-acetyl-α-d-galactopyranosyl bromide (5) (Scheme 2). The donor bromide 5 was obtained from D-galactose over five steps according to a reported procedure.²⁵ Donor 5 and glycosyl acceptor TPH 2 were coupled using silver carbonate and silver perchlorate to obtain 4-tert-butyldiphenylsilyloxyphenyl 2-azido-2-deoxy-3,4,6-tri-O-acetyl-α-d-galactopyranoside (6) in 64% yield with a 4:1 α:β ratio. Both isomers were carried forward for the next reaction. Deacetylation under Zemplén conditions afforded the intermediate triol. The triol was subjected to benzylidene acetal protection in the presence of p-toulenesulfonic acid (pTsOH) to afford 4-tert-butyldiphenylsilyloxyphenyl 2-azido-2-deoxy-4,6-O-benzylidene-α-d-galactopyranoside (7) in 53% yield over two steps. The two isomers were separated during purification, and only the α isomer was carried forward.

Scheme 2.

Synthesis of Intermediate Disaccharide 10

Acceptor 7 and donor building block A were condensed using NIS:TMSOTf at −40 °C to obtain disaccharide 8 in 63% yield (β-only).^26,27 Disaccharide 8 was subjected to BF₃∙OEt₂ and Et₃SiH at 0 °C to selectively open the benzylidene ring to afford acceptor 9 containing a free C-4 hydroxyl in 77% yield. The traditional method of benzylidene acetal ring opening with NaCNBH₃ and 1 N HCl in ether was initially attempted. However, it was unsuccessful in this case.

Acceptor disaccharide 9 was then coupled with thioglycoside building block B using NIS:TfOH in a 2:5 DCM:Et₂O mixture at −40 to −20 °C to produce trisaccharide 10 in 63% yield (α-only) (Scheme 3). The Boc-alanine residue was installed by reduction of azide 10 with Zn/AcOH followed by coupling with Boc-Ala-OH in the presence of propylphosphonic anhydride (T3P) and DIPEA to obtain trisaccharide derivative 11 in 56% yield. The Fmoc group was deprotected with a Et₂NH:DCM mixture, which was followed by Zemplén deesterification to remove the benzoyl esters; this gave trisaccharide 12 in 68% yield over the two steps. The benzyl groups were removed by hydrogenation using 10% Pd/C to obtain trisaccharide 13 in 86% yield. The deprotected compound was purified using a C18 silica gel column to afford trisaccharide 13. The TPH group was utilized as a purification handle since it was helpful for retention of the compound on the C18 column. In addition, the group locked the anomeric center for easy characterization. The TPH was removed via CAN oxidation in CH₃CN, which was followed by size-exclusion chromatography (Sephadex LH-20) to obtain the final target trisaccharide 14 with a free reducing end in 80% yield after 6 h.^28,29 It is notable that the reducing-end deprotection was achieved in a single step without removal of the TBDPS group.

Scheme 3.

Synthesis of the Common Outer-Core Trisaccharide Component of P. aeruginosa LPS

The disaccharide intermediate 9 was used to prepare the tetrasaccharide fragment of glycoform I. The coupling between acceptor 9 and glycosyl donor C was achieved using the NIS:TfOH promoter system to procure trisaccharide 15 in 45% yield^26,27 (Scheme 4). The levulinate ester was deprotected using hydrazine acetate in a DCM:methanol (4:1) mixture to access trisaccharide acceptor 16. Acceptor 16 and rhamnoside donor D were coupled using the NIS:TMSOTf promoter system to afford tetrasaccharide 17 in 57% yield.²⁶ Azide 17 was reduced using Zn/AcOH and coupled with Boc-Ala-OH in the presence of T3P to afford tetrasaccharide 18 in 68% yield.

Scheme 4.

Synthesis of the Common Outer-Core Tetrasaccharide Component of P. aeruginosa LPS

In summary, we have developed a flexible strategy to assemble oligosaccharides from the outer core domain of P. aeruginosa LPS using Lev- and Fmoc-protected thioglycosides. The hydroquinone-derived reducing-end capping group TPH was used for the first time in a complex oligosaccharide synthesis and was effective as a purification handle during reversed-phase and size-exclusion chromatography. The TPH group was remarkably stable toward many protection–deprotection chemistries and common glycosylation conditions. No electrophilic substitution on the hydroquinone in the presence of NIS was observed. The TPH could be efficiently removed in a single step via CAN oxidation without TBDPS deprotection to obtain free-reducing-end oligosaccharides such as 14. The presence of a latent hydroquinone moiety at the reducing end of a polysaccharide such as 13 should be useful for direct and mild conjugation chemistry with peptides or lipids to assemble glycoconjugates such as glycopeptides or glycolipid vaccines^30–32 using the latent phenolic hydroxyl group or the electron-rich ortho carbon as a chemoselective reaction center.